Distinct responses of different populations of bone cells to mechanical stress.

Abstract

To explore lineage-dependent responses to mechanical stress in bone cells, newborn rat calvarial cells, exhibiting differential characteristics of osteoblastic and osteocytic cells, were compared in their immediate and late responses to stretching. Seven fractions of sequentially prepared cells were cultured on Matrigel to promote their differentiation. By cyclically stretching the flexible bottom of culture plates, cells were exposed to a physiological stress of approximately 4000 microstrain on Matrigel. Cells in fractions IV, V and VI exhibited striking responses; the levels of cAMP and insulin-like growth factor I, bone Gla protein, and mineral accumulation were significantly elevated in the stretched cells. Also, proliferation was significantly inhibited regardless of the presence of 10(-6)M indomethacin. In contrast, osteoblasts in fraction III and osteocyte-like cells in fraction VII exhibited no significant response. Thus, these intermediate cells, very mature osteoblasts to young osteocytes, are likely to serve as a mechanosensor in bone, controlling the metabolic aspects of physical stress. We conclude that the responses of these young osteocytes to low level, physiological strain are transmitted in a manner different from the responses of osteoblasts to higher magnitude of strain in which PGE2 induces cell proliferation, as reported by others.

Parathyroid Hormone-activated Volume-sensitive Calcium Influx Pathways in Mechanically Loaded Osteocytes

“…This result indicates that osteocytes are more sensitive than endothelial cells, in which 20% elongation on silicon membrane caused little change in [Ca 2ϩ ] i (39). Osteocytes appear to be extremely sensitive to mechanical strain since our previous study of anabolic reaction showed that young osteocytes respond to low levels of mechanical strain (up to 4,000 E) to which osteoblasts do not respond (5). It is quite likely that the development of extensive cell processes, which is accompanied by differentiation of osteocytes, allows these cells to acquire sensitivity to stretching by the SA-Cat concentrated along the processes, as we presented in this article.…”

“…Rat Osteocytes-Rat osteocytes were prepared from frontal and parietal bones that were dissected aseptically from newborn rats according to the method of Mikuni-Takagaki et al (5) with some modifications. Briefly, pieces of bone were stripped of periosteal soft tissue and sutures and digested with 25 ml of 0.75 mg/ml collagenase.…”

“…Rat osteocytes cultured on Matrigel were placed 5-7 mm from the edge of the flexible plate wells coated with type I collagen (5,8). For stretching at the physiological level, the bottom of the plate was deformed by a computer operated vacuum system (Flexercell Strain Unit, Flexercell Corp., McKeesport, PA), with a frequency of 1/2.5 Hz (4 cycles of stretching in 10 s, followed by 50 s of relaxation) for 1 h. The expected strain was between 2,000 E (microstrain) and 4,000 E (0.4% elongation).…”

“…Direct inhibition of osteoclastic activity by osteocytederived protein through osteoclast-osteocyte attachment was demonstrated (2,3). Furthermore, intermittent mechanical loading within the physiological range enhances IGF-1 1 mRNA expression in osteocytes (4,5), suggesting that these cells transduce signals induced by mechanical stress to osteoblasts. Although various other mediators such as prostaglandins (6 -8), cyclooxygenase-2 (encoding prostaglandin G/H synthase), (8,9), endothelial, constitutive nitric oxide synthase (10), or c-fos (8,(11)(12)(13) have been suggested, the exact mechanosensing mechanisms in these cells are not clear.…”

Protein Kinase G and Focal Adhesion Kinase Converge on Src/Akt/β-Catenin Signaling Module in Osteoblast Mechanotransduction

“…Osteoblasts and osteocytes differ in their sensitivity to FSS or stretching and demonstrate different kinetics of ␤-catenin nuclear translocation, but both cell types respond to mechanical stimulation with an increase in intracellular calcium and NO production (53)(54)(55)(56)(57). Nuclear ␤-catenin together with its binding partner Lef1 regulates genes in osteoblasts and osteocytes necessary for maintenance of normal bone mass in vivo, including cox-2, and control osteoblast as well as osteoclast differentiation and survival (27-30, 41, 58).…”